I am not an expert in all aspects of this subject, but it has been my impression that, if the direction of unification theory is correct, all phenomena will eventually be traced to some aspect of the gravitational field itself, in other words, some property of our 4-dimensional space-time continuum. Virtual particles will be 'seen' as quantum fluctuations in the geometry of space-time as it changes from one topology to another at the Planck scale of 10^-33 centimeters. Presumably, the comings and goings of virtual particles are symptomatic of how the geometry of space-time changes. Since the geometry of space-time is, according to general relativity, just another name for what we experience as the gravitational field, all roads eventually lead to this font as their ultimate origin. Why nature has decided that space-time should be subject to quantum uncertainty is something that remains to be experimentally demonstrated. Until then, we have no provable explanation for where virtual particles get their energy during the time that Heisenberg's Uncertainty Principle lets them do their stuff!